BACKLIGHTING DEVICE

Abstract: A backlighting arrangement comprises a discharge lamp envelope (10) and a spirally formed inductive winding (2), the winding formed in a single layer. The electromagnetic field generated when an alternating voltage is supplied to the winding (2) couples into the envelope (10) wherein a plasma arc is established.

Description: BACKLIGHTING DEVICE. This invention relates to backlighting, and it relates especially, though not exclusively, to the backlighting of liquid crystal displays (LCDs) . One of the major advantages which LCD technology has over the much longer established and largely competitive cathode ray tube (CRT) technology is its compactness in the thickness dimension, and there are certain display applications for which the aforesaid compactness makes LCDs the only viable solution. There are numerous other applications in which the bulk of CRTs renders them undesirable for aesthetic reasons. Whereas CRTs effectively comprise their own light source, however, LCDs require external illumination. Sometimes this can be provided by ambient illumination, where the LCD is reflective in nature. More often, however, the LCD requires to be backlit and it is essential then that the backlighting technology does not unduly compromise the aforementioned compactness in the thickness dimension. The backlighting also needs to be of substantially uniform brightness, be economical in its power consumption and exhibit acceptable longevity. Considerable effort has thus been expended on the development of efficient and compact backlights for LCDs and backlights comprising fluorescent tubes and diffusers are well known. Moreover proposals for improved devices have been published, for example, in JP-A-63150850, JP-A- 63208543, JP-A-62150647 and US-A-4 , 872 , 741. All of the foregoing publications disclose relatively thin backlights which utilise electrodeless lamp technology. JP-A-63150850 and JP-A-63208543 disclose the use of several tubular electrodeless lamps in a common housing; the lamps being excited by different winding configurations. JP-A-62150647 and US-A-4, 872, 741 on the other hand disclose the use of electrodeless lamps comprising unitary flat envelopes. It is clearly desirable to use electrodeless technology for backlighting LCDs, because it is capable of delivering light outputs exhibiting good uniformity and high brightness, as well as exhibiting longevity and having the potential for compactness in the thickness dimension. Difficulties exist, however, in that electrodeless lamps contain a gas fill which needs to be excited in order for the lamp to operate and the striking and running of such lamps requires that radio frequency (RF) energy be applied to exciter elements and coupled into the gas fill. The efficiency of such coupling is of prime importance if the lamps are to run reliably and with high and uniform brightness. It is this aspect of electrodeless backlighting technology with which the present invention is principally concerned. JP-A-62150647 proposes that a coupling element be disposed around the periphery of a rectangular, electrodeless backlight. This arrangement, however, would not be capable of exciting the lamp with the required degree of uniformity once certain rather small dimensions in the plane of the backlight were exceeded. US-A- 4,872,741, on the other hand, proposes the use of an array of toroidal coils distributed over the back surface of a rectangular backlight. This arrangement, however, may give rise to limitations in striking the lamp and moreover it compromises to some extent the compactness of the assembly because the thickness of the toroidal coils adds to the overall thickness dimension of the backlight. Moreover, there tends to be a lack of control over the shape of the gas discharges generated, and thus limited uniformity performance. According to the invention there is provided an electrodeless backlight for illuminating a display, the backlight comprising a substantially flat assembly, configured in two dimensions to conforrr, to the dimensions of the display and with the third dimension representing the thickness of the assembly, incorporating at least one envelope containing a gaseous fill capable of responding to the application thereto of radio frequency (RF) energy to generate electromagnetic radiation which in turn stimulates a conversion agency to generate visible light and characterised by means for applying said RF energy to said gaseous fill including one or more drive means consisting of electrically conductive material conforming to a predetermined pattern having extent in said two dimensions but extending for substantially only the thickness of said electrically conductive material in said third dimension; said pattern being of generally spiral configuration and being juxtaposed with the said envelope. In order that the present invention may be clearly understood and readily carried into effect, certain embodiments thereof will now be described by way of example only with reference to the accompanying drawings of which: - Figure 1 illustrates schematically a first embodiment of the present invention. Figure 2 illustrates schematically alternative spirally formed windings; Figure 3 illustrates schematically the effect on the plasma arc of alternative windings; Figure 4 illustrates schematically alternative arrangements of spirally formed windings; Figure 5 illustrates schematically a circuit for driving a winding in accordance with the present invention; Figure 6 illustrates schematically a non-planar arrangement in accordance with the present invention; Figure 7 illustrates schematically a spiral winding formed directly upon a lamp envelope; Figure 8(a), 8(b) and 8(c) show how a backlight for a large area display can be built up from a number of individual, relatively small backlights each with its own spiral electrode or electrodes; Figure 9 illustrates a portion of a backlighting arrangement comprising a tesselation of hexagonal backlights; Figures 10 and 11 show, in perspective and exploded views respectively, a typical construction of a circular backlight in accordance with an embodiment of the invention; and Figure 12 illustrates a convenient circuit arrangement for an RF drive circuit. Referring firstly to Figure 1 it can be seen that an inductive winding 2 of electrically conductive material, in this case copper, is spirally formed upon a circuit board substrate 4. The circuit board 4 is initially coated on at least one of its major surfaces with copper, and the copper is removed, by a standard lithograph technique, to create the generally spiral pattern of the winding 2. The board 4 is brought into close proximity with a sealed glass envelope 10 containing a gas fill. The nature of the gas fill is determined by the desired optical characteristics to be produced by the backlight, but typically the envelope 10 contains an inert gas or gases at a reduced pressure (= 2mbar) and also contains a small amount of mercury. One of the two major surfaces of the envelope in this case that major surface which is furthest from the board 4 (ie, the front surface which would face the LCD to be illuminated) , bears a phosphor coating on its inside. When the board 4 and the envelope 10 are adjacent each other, as illustrated by the dotted line 12 in Figure 1, and with the winding 2 thus juxtaposed with the envelope 10, an alternating voltage at radio frequency (RF) is supplied to the winding 2 via the contact points 6, 8 from a voltage source (not shown in this figure) .The spiralling winding 2 need not be unidirectional as shown in the figure, but it is important that the currents induced thereby within the gas fill are non-cancelling. This requirement will be readily understood by those skilled in the art. As is known, a plasma arc is set up within the envelope 10 which generates ultraviolet radiation which is of a wavelength such that it excites the phosphor to emit visible radiation. In this manner the backlight produces visible light which may then be used to illuminate an LCD. Clearly the other major surface of the envelope 10 (as well as minor surfaces if desired) can be phosphor coated. Usually, the envelope 10 is made of glass, in which case the phosphor coating (or coatings) need to be internal of the envelope due to the fact that glasses absorb UV radiation and thus the efficiency of the device would be significantly reduced if the phosphor were external of the envelope 10. If, however, the envelope 10 in constructed of a material such as quartz which does not significantly absorb UV radiation, external phosphor coating, which is easier to implement than internal coatings, can be used. In any of the arrangements described, reflective material may be provided on or adjacent the rear surface of the envelope 10 to reflect back into the envelope any visible or ultra violet radiation that would otherwise have escaped through the rear surface and thus have been wasted. In the example of Figure 1, the winding 2 is shown as being spirally formed as a single, regular geometric spiral. By reference also to Figure 2 it can be seen that alternative forms of spiralling for the winding are possible, each of which offers different advantages over known non-spiralling inductive windings. A consistent advantage, however, is that conformity with such generally spiral patterns renders the striking (ie., starting) of the discharge easier and more reliable. This is attributed to the radial nature of the electric fields generated by means of such generally spiral patterns. It can be seen that Figure 2(a) illustrates the same circular spirally formed windings 2 of Figure 1 whilst Figure 2 (b) illustrates an elliptical spirally formed winding 2, Figure 2(c) a rectangular winding and Figure 2 (d) a multiple concentric winding. It will thus be apparent that the terms "spirally" and "spiral" as used here throughout are intended to cover not only curved but linear, non-uniform and plural spirals. A characteristic of each however is an ever increasing "radius" from a central point of reference with each complete loop or turn of the winding. Although the shape of the spirally formed winding 2 is to a degree arbitrary, as illustrated in Figure 2, each shape produces its own characteristic plasma arc within the gas fill of the envelope 10. By referring to Figures 1, 2 and 3, several such characteristics will be illustrated. In each of Figures 3(a) , (b) and (c) a section ABCD of envelope 10 is illustrated as viewed in the direction of the arrows in Figure 1, such that the line CD bisects the envelope along a side of greatest length. Each of the figures in Figure 3 show examples of the plasma arch discharge within the gas fill for different shapes of winding 2. Thus the solid lines 13 within the sections ABCD are electron density contours of the plasma arc. Figure 3 (a) illustrates the non-uniform discharge produced by a toroidal coil, as proposed in US-A- 4,872,741. The coil has a circular shape and is shown in cross section, referenced as 12. In Figure 3 (b) , the cross-section 14 is that of the regular spirally formed winding 2 such as is illustrated in Figure 2 (a) . It can be seen that the high electron density contours 13 of Figure 3 (b) are more evenly distributed around the envelope than those of Figure 3(a) . This is due tc the flat spiral nature of the winding. Similarly the cross section 16 of Figure 3(c) is that of the spirally formed winding 2 of Figure 2(d) . Th s, as can clearly be seen by the contour shapes, can in principle provide the most uniform electron density distribution, and hence most uniform light output, of all the winding arrangements shown in Figure 3. In practice, however, it is preferred to use either the circular spiral arrangement of Figure 2 (a) or the elliptical configuration of Figure 2 (b) . Although it is a feature of the present invention that a spirally formed winding in accordance with the present invention be in a single layer - thus effectively providing a two-dimensional winding which occupies less space than toroidal windings for example, it is possible for many such layers to be stacked and/or interleaved as illustrated in Figure 4. Figure 4(a) illustrates many square spirally formed windings 18, whilst Figure 4(b) illustrates many hexagonal spirally formed windings 20. As has been discussed above with particular reference to Figure 3, each of these arrangements will impart its own characteristic to the plasma arc within envelope 10. In Figure 4(c), which illustrates circular spirally formed windings, the windings 22 in the foreground are formed on one side of board 4 whilst the windings 234 are formed on the other side of board 4. Referring now to Figure 5, circuitry suitable for driving a winding in accordance with the present invention is schematically illustrated. The winding 2 is coupled, by contact points 6, 8 and via capacitors 26, 28 to a voltage generator 3Q. The generator 30 produces an alternating voltage at 13.56 MHz and the capacitor 26 is a 19pF capacitor whilst capacitor 28 is 54pF. The winding 2 is dimensioned (in a known manner) to have an inductance of 2.5mK. Although a spirally formed winding in accordance with the present invention must be formed in a single layer, there is no necessity for this layer to be planar, nor for the envelope to be planar either, as in the previous examples. Naturally, for efficient coupling between the backlight and the winding, then the shape of the single layer of the winding should conform to that of the backlight, so this layer may curve or twist as appropriate. Such an example is shown in Figure 6 in which like components to those described herebefore are similarly numbered. In Figure 6 it can be seen that, whilst the winding 2 is still in a single layer on board 4, the board 4 is curved rather than planar, as previously shown. In the above examples, the winding 2 is shown formed on board 4 and is separate from, although adjacent the envelope 10. This need not necessarily be so and, as Figure 7 illustrates, the winding 2 can be formed on and integral with the envelope 10. This provides an even slimmer arrangement than has been described above. Those skilled in the art will appreciate that there is no compulsion for the spirally formed winding - whatever its shape - to possess a uniform turn spacing. The present invention thus aims to achieve thinner backlights with a more uniform light output than has hereto been known due to employment of single-layer spirally-formed windings. It will also be apparent to those skilled in the art that a spirally formed winding in accordance with the present invention will always have a gap or spacing between each successive turn or loop of the electrically conductive member. This gap or spacing may be achieved by, for example, leaving an air gap between each loop or by coating the conductive member with an electrically insultative material and then winding this amalgam around itself in a spiralling manner. It is convenient, when constructing a backlight for a relatively large area display (eg., 6 inches x 8 inches), to build this up out of several smaller backlights, each with its own energising spiral electrode or electrodes. For example, a 6" x 8" backlight may be made up of four 5" x 2" envelopes placed side-by-side and each driven by one, two or three spiral electrodes in configurations as shown in Figures 8(a) , 8(b) and 8(c) . Clearly, the envelopes need not be rectangular, and if they are there is no need for them to conform to the configuration shown in Figures 8(a), 8(b) and 8(c) . For instance it may be preferred to construct the backlight for a 6" x 8" display from eight 3" x 2" envelopes, rather than four 6" x 2" envelopes, and the configuration of the spiral electrodes might differ accordingly. In general, however, it is preferred to minimise the number of individual envelopes used to construct a composite backlight of the kind discussed in this and the preceding paragraph, since the junctions, or lines of abutment, between the various envelopes can cause non-uniformities in the light output. This can be compensated for if desired by means of various optical components, such as diffusers or microlens arrays, but it is cheaper and easier to manufacture composite backlights without extra components. A composite backlight made up of a tesselation of individual hexagonally-shaped envelopes creating a honeycomb-like structure is a particularly convenient configuration in that the regular yet non-linear abutment lines are so distributed that non-linearities in the light output are distributed in an acceptable fashion over the entire surface area of the display. Such an arrangement is shown schematically in Figure 9. In all embodiments it is convenient, though not essential, to construct the spiral electrodes by etching through a copper layer clad onto a non-conductive substrate, such as a printed circuit board. The thickness of the copper is typically 30 to 60 microns and the etching is carried out in accordance with standard photolithographic procedures. Since the spiral electrodes are typically driven at 13.55 MHz, and supplied w th about twenty watts of power, the substrate material must be able to withstand significant heating and it may be preferred in some circumstances to use a ceramic substrate. Alternatively, as mentioned previously, the spiral electrode or electrodes can be formed directly on the surface of the envelope itself, and this envelope will typically be made of a glass or quartz. It will be observed from Figure 5 that capacitors are, in this case, used in order to tune the circuit with the inductance of the spiral electrode. It is convenient for these capacitors to be formed in unused areas of the copper cladding, away from the area occupied by the spiral electrode or electrodes, especially when using a substrate which is copper clad on both sides. In that case, overlying areas of the copper cladding on opposite sides of the substrate can be left in place to create capacitors. In any event, it is convenient for a fine tuning capability to be provided, and this can take the form of trimming capacitors of movable interleaving elements, suitably placed in relation to the power supply and the electrode or electrodes. Figure 10 shows a typical construction for a 70mm diameter circular backlight unit, this comprising in essence a main housing 34 containing a squat cylindrical backlight envelope 36. As can be seen more easily in the exploded view of Figure 11, the components 34 and 36 are supplemented by, at the rear, a spiral electrode 38 of the kind hitherto described, and d.c to RF converter and matching power drive circuitry 40, which is located within the main housing 34. In addition, there are various location and protective components such as a rear clamping plate 42 and unit back piece 44 and, at the front of the unit, a front clamping plate 46 neoprene cushioning shim 48, transparent protective screen 50 and envelope retaining plate 52. A model and serial number plate 54 is also affixed to the front of the main housing 34. DC power is applied via a socket 56 in the main housing 34 and connects to the circuitry 40. Figure 12 illustrates a convenient circuit arrangement for a 13.56 MHz, 20 watt source to drive a spiral electrode as described hereinbefore. This circuit arrangement is intended to be housed in a die cast box (not shown) built to be used with the circular lamp described earlier in relation to Figures 10 and 11 (not shown) . It requires a DC power supply of up to 30 volts with a current capability of 1.25 amps. A heat sink fixed to the outside of the box allows continuous operation. The circuit has a crystal controlled oscillator 58 driving a single ended amplifier 60. This amplifier is designed to be operated in Class E with a 50 ohm load impedance when a DC to RF efficiency of 80% is obtained. Lower efficiencies may be obtained when driving other impedances. Two FETs62 and 64 are used in the circuit, a 2N7000(62) in the oscillator and an IRF510(64) as the output amplifier. The initial bias for the 2N7000 is provided by a 1 /100K ohm potential divider 66, 68 providing a gate voltage. A supply voltage of about 25 is required to start the oscillations which, once started, override the starting bias so that the supply can be reduced if required. The feedback to maintain the oscillations is provided by a 4K7 resistor 70 and 13.56 MHz crystal 72 between the drain gate of the FET 62. The phase of the feedback signal is optimised by a 33pF capacitor 74 connected to ground. The tuned load for this stage is provided by a 0.68 uH inductor 76, 220 pF capacitor 78 and gate capacity of the FET 64. For the required output, a load impedance of about 10 ohms is required so a low pass "1" section impedance transformer is incorporated. The series inductance of this transformer is combined with the series inductor of the output network to give a single component. This inductor is made variable to allow for circuit adjustment and tolerances. A lυnF feed through capacitor 80 and 100 nF capacitor 82 from supply to ground reduce the RF conducted along the supply wires. Although a class E circuit is simple and efficient it imposes high voltages and currents on the transistor. The peak current is nearly three times the mean and the peak voltage over three times the supply. With a 100 volt transistor being used the maximum supply should be limited to 30 volts. For completeness, the values of the remaining components in the circuit illustrated in Figure 12 are as follows: Resistor 84 1M Resistor 86 100k Resistor 88 120 Capacitor 90 4n7 Resistor 92 Ik Inductor 94 4μH Capacitor 96 150p Capacitor 98 470p Inductor 100 0.6μH variable Capacitor 102 180p Capacitor 104 330p A further refinement, useful in circumstances where very high brightness and uniformity of illumination are required, will now be described, in which the front phosphor coating (i.e. that on that major surface of the backlight which faces the display) can be omitted. This layer is particularly difficult to produce as it must be sufficiently thick to effect useful conversion of the forward - directed ultra violet radiation, from the discharge, into visible light whilst not being so thick that it absorbs too much of the forward - directed visible light generated by the real phosphor. In practice achieving both these objectives simultaneously is very difficult and a compromise has to be reached which involves using a phosphor too thin to fully convert into visible radiation all the U.V. reaching the front phosphor. Ultimately this may mean that the backlight does not achieve its full potential in terms of brightness or uniformity. In addition producing envelopes with a thick rear phosphor and well controlled thin front phosphor results in significant manufacturing difficulties, particularly if a glass envelope with internal phosphors is to be used. These difficulties can be overcome or reduced if the backlight is provided with only a single phosphor layer which is at the rear of the envelope and may be external or internal thereof. This phosphor layer is sufficiently thick to ensure maximum generation of visible radiation in the forward direction (i.e. towards the display). A dichroic mirror is placed adjacent the front surface of the envelope (which must be U.V. transmissive) , and this mirror has the properties of transmitting radiation in the visible part of the spectrum (wavelength > 400 nm) and reflecting radiation in the U.V. part of the spectrum (wavelength < 400 nm, but principally the main U.V. line near to 254 nm which is generated in low pressure mercury discharges) . This results in useful visible radiation which is generated both from the rear surface phosphor and directly from the plasma being transmitted in the forward direction from the backlight with minimal attenuation. However, forward directed U.V. radiation from the plasma is reflected back towards the plasma where it is re- absorbed in the plasma or propagates through the plasma and helps to irradiate the rear surface phosphor. In this way the major proportion of the forward directed U.V. radiation from the plasma will be made use of, either optically pumping the discharge plasma (and therefore reducing the RF Power needed to drive the discharge) or increasing the amount of visible radiation generated from the rear phosphor. Dichroic mirrors having the required properties of very high transmission in one part of the spectrum and high reflectance in another part of the spectrum are currently available.

Claims: CLAIMS 1. An electrodeless backlight for illuminating a display, the backlight comprising a substantially flat assembly, configured in two dimensions to conform to the dimensions of the display and with the third dimension representing the thickness of the assembly, incorporating at least one envelope containing a gaseous fill capable of responding to the application thereto of radio frequency (RF) energy to generate electromagnetic radiation which in turn stimulates a conversion agency to generate visible light and characterised by means for applying said RF energy to said gaseous fill including one or more drive means consisting of electrically conductive material conforming to a predetermined pattern having extent in said two dimensions but extending for substantially only the thickness of said electrically conductive material in said third dimension; said pattern being of generally spiral configuration and being justaposed with the said envelope. 2. A backlight according to Claim 1 wherein said envelope is made of a glass and bears an internal phosphor coating on at least a surface thereof extending in said two dimensions and facing said display. 3. A backlight according to Claim 1 wherein said envelope is made of ultra-violet transmissive material and bears an external phosphor coating on at least a surface thereof extending in said two dimensions and facing said display. 4. A backlight according to any preceding claim wherein said predetermined generally spiral pattern conforms to a substantially circular configuration. 5. A backlight according to any of claims 1, 2 or 3 wherein said predetermined generally spiral pattern conforms to a substantially elliptical configuration. 6. A backlight according to any of claims 1, 2 or 3 wherein said predetermined generally spiral pattern conforms to a substantially rectangular configuration. 7. A backlight according to any preceding claim wherein said predetermined generally spiral pattern contains at least one irregularity in the radial spacing between successive turns thereof. 8. A backlight according to any preceding claim wherein said electrically conductive material is copper of thickness in the range 30 microns to 60 microns. 9. A backlight according to Claim 8 wherein said copper comprises a cladding on a non-metallic substrate. 10. A backlight according to Claim 9 wherein at least one capacitor used in applying said RF energy to said drive means is constituted by a part of said copper cladding. 11. A backlight arrangement comprising an assembly of backlights according to any preceding claim. 12. An arrangement according to Claim 11 wherein each backlight in the assembly comprises a hexagonal envelope.

Inventors: ALLEN PHILIP CHARLES, ALLEN, PHILIP, CHARLES
Applicants: ALLEN PHILIP CHARLES, CENTRAL RESEARCH LAB LTD, ALLEN, PHILIP, CHARLES, CENTRAL RESEARCH LABORATORIES LIMITED
Application year: 1994
Main classification: H01J65
IPC codes: F21V8, H01J65